American Trees Are on the Move But No One Knows Why

North American forests are moving out. Deciduous, broadleaf species are moving west. Evergreen, needle-leaf species are moving north. The reason? Climate change.

A study of forest populations published in Science Advances has found that the mix of trees long associated with each region is changing rapidly. Trees that once thrived in specific areas are dying in those regions, and are appearing in new areas that now feature the rainfall amounts and temperatures they require. Tree species, like animals species, are on the move in search of suitable habitat as conditions change.

As equatorial zones increase in temperature, growing seasons are extending northwards. Trees that favor cooler conditions are finding their traditional zones too hot, and their seeds are now taking hold in northern regions that were formerly too cold — but are now just right.

Climatologists and backyard gardeners alike have noted this change in recent years, and in 2012, the U.S. Department of Agriculture updated its Plant Hardiness Map to reflect the shifts in growing zones to match the current warmer reality. However, scientists were surprised by data indicating that tree species are not just moving north, but moving west. As former dry zones receive greater rainfall amounts, trees that flourished in southern and eastern regions are finding new purchase in soils to the west.

“That was a huge surprise for us,” said Songlin Fei, a professor of forestry at Purdue University and one of the authors of the study.

“Different species are responding to climate change differently. Most of the broadleaf species—deciduous trees—are following moisture moving westward. The evergreen trees—the needle species—are primarily moving northward,” said Fei. Hardly any types of trees are moving south or east.

The study found that about three-quarters of tree species common to eastern American forests — including white oaks, sugar maples, and American hollies — have moved west since 1980. More than half of the species studied also moved northward during the same